Polyphase driving machines fed by a converter, such as three-phase synchronous machines for example, which are provided with a tachometer generator and a rotor position transmitter are commercially available. In these machines, a speed-proportional voltage is generated from the phase voltages of the tachometer generator and fed to a speed controller with subordinated current control. The rotor position transmitter generates a rotor position signal, also called a commutation signal, which is fed to a selection logic circuit of the converter. If at any time at least one phase of the polyphase tachometer generator fails due to a short-circuit in the tachometer generator, the speed-proportional voltage is formed only from the phase voltages of the phases minus at least one phase. Thus, the formed actual speed value will no longer agree with the actual speed value of the driving machine. This can result in control difficulties which lead to a situation where the machine performing work must be turned off. In addition, the tachometer generator is heated up by a short-circuit or short-circuit current therein. If this machine is installed in a room containing an explosion hazard the surface temperature of the tachometer generator must not exceed a predetermined temperature. In this situation the machine performing work must be turned off.
A problem then is to provide a method and circuit arrangement for monitoring the phase-shifted phase voltages having a constant range of a polyphase tachometer generator in order to detect phase failures, so that a false indication of the actual speed value and a heating of the surface of the tachometer generator are prevented.